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CHAPTER 2 LITERATURE REVIEW The main difference between YSZ and pure zirconia lies in its phase transformation, the phase diagram is shown in Figure 2.3. For pure zirconia, two different phases are formed during thermal cycles. When the temperature is located between room temperature and 1170 °C, zirconia adapts in monoclinic phase. The phase transformation to tetragonal occurs when the temperature is in the range between 1170 °C and 2370 °C, and the phase turns to cubic when the temperature is between 2370 °C and 2680 °C (melting point) which means phase transformation will happen when the engine is in operation [12]. However, when the phase transformation happens, there is a 4 to 6% of volume change occurred, which is extremely harmful to the lifetime of top coat. In order to avoid the transformation, adding another element will stabilize the cubic phase of zirconia and commonly yttrium, magnesium, calcium, and gadolinium et al. are added. For aero-engines, zirconia with 7 to 8 wt.% of added Y2O3 to stabilize the tetragonal phase is used. The stabilizing effect of yttria is manifested in Figure 2.3 [12]. Actually, the thermal conductivity of YSZ could be lower by adding more Y2O3 but the lifetime of TBC would be sacrificed as well. This phenomenon is still unexplained [11,13,14]. When the temperature is below 1050 °C, YSZ is composed of cubic and monoclinic phase. When the temperature is higher than 1050 °C, monoclinic phase transforms into tetragonal phase. Modern techniques for YSZ coating are high rate and non-equilibrium processes which lead to its metastable tetragonal prime phase. Contributed by the tetragonal prime phase, TBC provides longer lifetime because it does not transfer to monoclinic phase Page 23PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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